1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate for an LCD device and a method of manufacturing the same to obtain a simplified manufacturing process.
2. Discussion of the Related Art
With the development of the information society, demand for various flat display devices having advantageous characteristics such as a thin profile, light weight, and low power consumption have increased. Among the various flat display devices, LCD devices having high resolution and high picture quality are applied for monitors of notebook and desktop computers.
In general, the LCD device includes lower and upper substrates facing each other. Respective surfaces of the two substrates having electrodes thereon are opposite to each other. Liquid crystal material is injected between the two substrates, and then liquid crystal molecules are driven according to an electric field generated by applying a voltage to the two electrodes of the respective substrates. Accordingly, it is possible to control light transmittance through the liquid crystal material by the electric field, thereby producing images.
The lower substrate of the LCD device is an array substrate including a thin film transistor for applying a signal to a pixel electrode. The lower substrate is formed by photolithography. Meanwhile, the upper substrate includes a color filter sequentially arranging red, green and blue colors, and a common electrode. The upper substrate is formed by a pigment dispersion method, a dye method, or a print method. In particular, the pigment dispersion method is most often used due to fineness and exceptional color realization.
The LCD device has an array substrate (lower substrate) and a color filter substrate (upper substrate), wherein the pixel electrode of the lower substrate is in one-to-one correspondence with the color filter of the upper substrate. When arranging the lower and upper substrates, there may be misalignment, thereby generating light leakage. In order to prevent this problem, the width of a black matrix formed on the upper substrate must be increased. However, doing so may lower the aperture ratio in the LCD device. Accordingly, a method of forming the color filter on the array substrate is proposed to prevent misalignment and to improve the aperture ratio in the LCD device. This method of forming the color filter on the thin film transistor is referred to as a COT (color filter on TFT) structure.
Hereinafter, an array substrate for an LCD device having a COT structure according to the related art and a method of manufacturing the same will be described as follows.
FIG. 1 is a cross-sectional view illustrating an array substrate for an LCD device having a COT structure therein according to the related art. As shown in FIG. 1, a gate electrode 12 is formed on a transparent first substrate 11. The gate electrode 12 is formed of a conductive material such as metal. Then, a gate insulating layer 13 of silicon nitride SiNx or silicon oxide SiO2 is formed on the transparent first substrate 11 including the gate electrode 12. Subsequently, an active layer 14 of amorphous silicon is formed on the gate insulating layer 13 above the gate electrode 12. Then, an ohmic contact layer 15 of amorphous silicon doped with an impurity is formed thereon. Source and drain electrodes 16a and 16b are formed overlapping with both sides of the ohmic contact layer 15. The source and drain electrodes 16a and 16b of conductive material such as metal are formed at a predetermined interval from each other. At this time, a thin film transistor T is formed of the source and drain electrodes 16a and 16b and the gate electrode 12.
Although not shown, the gate electrode 12 is connected to a gate line, and the source electrode 16a is connected to a data line. Also, the gate and data lines cross each other to define a pixel region. Then, a passivation layer 17 is formed on an entire surface of the first substrate 11 including the source and drain electrodes 16a and 16b to protect the thin film transistor T. At this time, the passivation layer 17 is formed of silicon nitride, silicon oxide, or an organic insulating layer. Next, a color filter layer 18 is formed in the pixel: region on the passivation layer 17, wherein the color filter layer 18 is formed by sequentially arranging red, green and blue colors. At this time, a contact hole 19 for exposing a predetermined portion of the drain electrode 16b is formed in the color filter layer 18 and the passivation layer 17. After that, a pixel electrode 20 of transparent conductive material is formed on the color filter layer 18. The pixel electrode 20 is electrically connected with the drain electrode 16b through the contact hole 19.
In the meantime, a transparent second substrate 21 is positioned above the transparent first substrate 11 at a predetermined interval. On a surface of the second substrate 21 facing the first substrate 11, there is a black matrix layer 22 corresponding to the thin film transistor T of the first substrate 11. Although not shown, the black matrix layer 22 has an opening corresponding to the pixel electrode 20 of the first substrate 11. Accordingly, the black matrix layer 22 blocks light from regions except the pixel regions of the first substrate 11. That is, the black matrix layer 22 prevents light leakage as liquid crystal molecules are tilted. In addition, the black matrix layer 22 prevents a leakage current in the thin film transistor T. Then, a common electrode 23 of conductive material is formed on an entire surface of the second substrate 21 including the black matrix layer 22. After that, a liquid crystal layer 30 is formed between the pixel electrode 20 and the common electrode 23. When voltage is applied between the pixel electrode 20 and the common electrode 23, the alignment direction of the liquid crystal molecules in the liquid crystal layer 30 is changed by an electric field generated between the pixel electrode 20 and the common electrode 23. Although not shown, alignment layers are formed on the common electrode 23 and the pixel electrode 20, so as to determine the initial alignment of the liquid crystal molecules.
Accordingly, when the color filter layer 18 is formed on the first substrate (lower substrate) 11, it is possible to prevent misalignment between the color filter layer 18 and the pixel electrode 20. Thus, the aperture ratio may be improved without increasing a width of the black matrix layer 22. Also, the color filter layer 18, formed on the first substrate 11, makes it possible to prevent misalignment between the color filter layer 18 and the pixel electrode 20 when bonding the first and second substrates 11 and 21 to each other. That is, it is possible to decrease a bonding margin of the black matrix layer 22 of the second substrate 21. If the barrier pattern is formed of a black matrix material preventing light transmittance, it is possible to omit the black matrix layer 22 of the second substrate 21, thereby improving the aperture ratio.
FIG. 2A to FIG. 2E are cross-sectional views illustrating manufacturing process steps of the array substrate for the LCD device according to the related art. The color filter layer is formed by the pigment dispersion method.
First, as shown in FIG. 2A, the metal material is deposited on the transparent first substrate 11, and then is selectively removed by photolithography using a first mask, thereby forming the gate electrode 12. When forming the gate electrode 12, the gate line (not shown) connected to the gate electrode 12 is formed in a first direction. Subsequently, the insulating material, such as silicon nitride or silicon oxide, is deposited on the entire surface of the first substrate 11 including the gate electrode 12, thereby forming the gate insulating layer 13. Then, the amorphous silicon layer and an amorphous silicon layer doped with an impurity are sequentially formed on the gate insulating layer 13, and then selectively removed by photolithography using a second mask, thereby forming the active layer 14 and the ohmic contact layer 15.
As shown in FIG. 2B, the metal material is deposited on the entire surface of the first substrate 11, and then selectively removed by photolithography using a third mask, thereby forming the source and drain electrodes 16a and 16b overlapped with both sides, of the ohmic contact layer 15 at a predetermined interval from each other. The data line (not shown) is simultaneously formed with the source and drain electrodes 16a and 16b, the data line extending from the source electrode 16a in a second direction. The data line is substantially perpendicular to the gate line, thereby defining the pixel region. Then, the ohmic contact layer 15 exposed by the source and drain electrodes 16a and 16b is selectively removed. At this time, the source and drain electrodes 16a and 16b are formed at a predetermined interval from each other to form a channel region. As a result, the thin film transistor T includes the source and drain electrodes 16a and 16b and the gate electrode 12.
As shown in FIG. 2C, the passivation layer 17 of silicon nitride, silicon oxide, or organic insulating layer is formed on the source and drain electrodes 16a and 16b. If the passivation layer 17 is formed of the organic insulating layer, a step difference generated by the thin film transistor T is removed to obtain smoothness and easiness in the following manufacturing process steps.
As shown in FIG. 2D, a photosensitive material is formed on the passivation layer 17, and then exposure and pattering process is performed thereto, thereby forming the color filter layer 18 in the pixel region. The color filter layer 18 and the passivation layer 17 are selectively removed on the drain electrode 16b, thereby forming the contact hole 19 exposing a predetermined portion of the drain electrode 16b. The color filter layer 18 is formed of red, green and blue colors, whereby the color filter layer 18 with various colors is formed by repeating deposition and exposure and developing process three times.
As shown in FIG. 2E, a transparent conductive material is deposited on the color filter layer 18, and then patterned, whereby the pixel electrode 20 is formed for being electrically connected with the drain electrode 16b through the contact hole 19.
However, the method of manufacturing the array substrate for the LCD device according to the related art has the following disadvantages.
If the color filter layer is formed by repetition of the exposure and developing process a number of times after deposition of the photosensitive material, it requires increased time to form the color filter layer. Also, in order to form the color filter layer, the photosensitive material is formed on the entire surface of the substrate by a spin-coating method, and then partially removed, thereby increasing consumption of materials. On the exposure and developing process after deposition of the photosensitive material, elements of impurity or developer of the photosensitive material may penetrate into the adjacent layer, whereby it may deteriorate characteristics of the thin film transistor. In order to prevent this problem, it is required to form the passivation layer on the thin film transistor.